1. Field
The present disclosure relates to tire treads and more particularly to treads comprising grooves comprising flexible devices for reducing the noise generated by air going into resonance in these grooves during driving.
2. Description of Related Art
It is known that as a tire that is running comes into contact with a road surface air is made to circulate in a pipe formed by a groove notably a groove of circumferential overall orientation, and the road surface itself, this pipe being open at both ends.
The air in this pipe forms a vibrating air column the resonant frequency of which is dependent on the length between the two ends of the pipe and therefore on the length of groove in contact with the road surface.
This resonance of the air in the grooves has the result of generating, in a vehicle fitted with these tires, a noise inside the vehicle and a noise outside the vehicle.
These inside and outside noises usually correspond to a frequency of 1 kHz or thereabouts, which is a frequency to which the human ear is particularly sensitive. In order to reduce such resonant noise it is known practice to arrange, in each circumferentially oriented or generally circumferentially oriented groove, a plurality of relatively thin closure membranes or blades made of a rubbery material, each closure membrane or blade occupying the entire cross section of the groove or at least a large proportion of this cross section. Each closure membrane may extend from the bottom of the groove or may be fixed to at least one of the walls delimiting the said groove. Relatively thin here means that each closure membrane is able to flex in order to open the cross section of the groove under a flow of liquid notably during wet-weather driving.
Thanks to these closure membranes, the length of the column of air in each circumferential groove is reduced by comparison with the total length of groove in the contact patch, and this leads to a change in the resonant frequency. The frequency is shifted towards resonant frequency values to which the human ear is less sensitive.
Of course, in order to maintain the water clearing function, when driving on a wet road surface, it is necessary for this membrane to be able to flex appropriately under the action of the pressure of the water and thus open the cross section of the groove. Various solutions of this type for reducing the resonance of the air column vibrating in the grooves have been proposed.
One approach is, for example, described in patent documents FR2715891, JP3-276802, JP08-150812, and JP4-221807.
One of the problems encountered with this type of blade fixed to the walls of the groove is the difficulty in achieving fullest closing-off of the cross section of the groove in order to reduce or even eliminate resonance of air in the groove while at the same time making these blades easy to mould and to release from the mould.
Definitions:
The tread surface of a tread corresponds to the surface that comes into contact with the ground when a tire fitted with such a tread is being driven on.
A groove corresponds to a space formed in a tread, the space being delimited by walls of material, these walls being joined together by a groove bottom distant from the tread surface by a distance equal to the depth of the groove. The groove bottom corresponds to that part of the groove that is situated radially underneath the tire wear indicator that indicates the minimum tread depth beyond which the tire tread no longer meets the legal requirements for use.
A block is a raised element formed on a tread, this element being delimited by voids or grooves and comprising lateral walls and a contact face, the latter—which is intended to come into contact with the road surface during driving, forming part of the tread surface of the tread.
A rib is a raised element formed on a tread, this element being delimited by two grooves. A rib comprises two lateral walls and a contact face, the latter being intended to come into contact with the road surface.
A radial direction means a direction perpendicular to the axis of rotation of the tire (this direction corresponds to the direction of the thickness of the tread).
An axial direction means a direction parallel to the axis of rotation of the tire.
A circumferential direction means a direction which is tangential to any circle centred on the axis of rotation. This direction is perpendicular both to the axial direction and to a radial direction.